Expanded PTFE Articles and Method of Making Same

ABSTRACT

Unique PTFE structures comprising islands of PTFE attached to an underlying expanded polytetrafluoroethylene (ePTFE) structure and to methods of making such structures is disclosed. The ePTFE material may or may not have been exposed to amorphous locking temperatures. These unique structures exhibit islands of PTFE attached to and raised above the expanded PTFE structures.

RELATED APPLICATIONS

The present application is a division of co-pending U.S. patentapplication Ser. No. 11/000,414 filed Nov. 29, 2004, which was based onU.S. Provisional Patent Application 60/605,127 filed Aug. 26, 2004, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to unique expanded PTFE articles. Morespecifically, it is directed to novel structures of expanded PTFE and anovel process for preparing the structures.

BACKGROUND OF THE INVENTION

The structure of expanded PTFE (“ePTFE”) is well known to becharacterized by nodes interconnected by fibrils, as taught in U.S.Patent Nos. 3,953,566 and 4,187,390, to Gore, and which patents havebeen the foundation for a significant body of work directed to ePTFEmaterials. The node and fibril character of the ePTFE structure has beenmodified in many ways since it was first described in these patents. Forexample, highly expanded materials, as in the case of high strengthfibers, can exhibit exceedingly long fibrils and relatively small nodes.Other process conditions can yield articles, for example, with nodesthat extend through the thickness of the article.

Surface treatment of ePTFE structure has also been carried out by avariety of techniques in order to modify the ePTFE structure. Okita(U.S. Pat. No. 4,2308,745) teaches exposing the outer surface of anePTFE tube, specifically a vascular prosthesis, to a more severe (i.e.,higher) thermal treatment than the inner surface in order to effect afiner structure on the inside than on the outside of the tube. One ofordinary skill in the art will recognize that Okita's process isconsistent with prior art amorphous locking processes, the onlydifference being preferential exposure of the outer surface of the ePTFEstructure to greater thermal energy.

Zukowski (U.S. Pat. No. 5,462,781) teaches employing plasma treatment toeffect removal of fibrils from the surface of porous ePTFE in order toachieve a structure with freestanding nodes on the surface which are notinterconnected by fibrils. No further treatment after the plasmatreatment is disclosed or contemplated in the teachings.

Martakos et al. (U.S. Pat. No. 6,573,311) teach plasma glow dischargetreatment, which includes plasma etching, of polymer articles at variousstages during the polymer resin processing. Martakos et al. distinguishover conventional processes by noting that the prior art techniquesoperate on finished, fabricated and/or finally processed materials,which are “ineffective at modifying bulk substrate properties, such asporosity and permeability.” Martakos et al. teach plasma treating at sixpossible polymer resin process steps; however, no such treatment with orsubsequent to amorphous locking is described or suggested. Again, thefocus of Martakos et al. is to affect bulk properties such as porosityand/or chemistry quality in the finished articles.

Other means of creating new surfaces on porous PTFE and treating thesurface of porous PTFE abound in the prior art. Butters (U.S. Pat. No.5,296,292) teaches a fishing flyline consisting of a core with a porousPTFE cover that can be modified to improve abrasion resistance. Abrasionresistance of the flyline is improved by modifying the outer covereither through adding a coating of abrasion resistant material to it orby densifying the porous PTFE cover.

In a further example, Campbell et al (U.S. Pat. No. 5,747,128) teach ameans of creating regions of high and low bulk density throughout aporous PTFE article. Additionally, Kowligi et al. (U.S. Pat. No.5,466,509) teach impressing a pattern onto an ePTFE surface, and Seileret al. (U.S. Pat. No. 4,647,416) teach the scoring PTFE tubes duringfabrication in order to create external ribs.

However, none of the prior art references teach applicants' uniquecombination of processing to create a unique surface on PTFE which hasheretofore not been seen.

SUMMARY OF THE INVENTION

The present invention is directed to a unique PTFE structure comprisingislands of PTFE attached to an underlying expandedpolytetrafluoroethylene (ePTFE) structure and to methods of making sucha structure. The ePTFE material may or may not have been exposed toamorphous locking temperatures. These unique structures exhibit islandsof PTFE attached to and raised above the expanded PTFE structures. By“raised” is meant that when the article is viewed in cross-section, suchas in a photomicrograph of the article cross-section, the islands areseen to rise above the baseline defined by the outer surface of theunderlying node-fibril structure by a length, “h.” Referring to FIG. 1,which shows a cross-section of an expanded PTFE fiber 10 with island 12,the height of the island 12 rises a height “h” above the surface 14, or“baseline,” of the underlying ePTFE structure.

These raised regions, or islands, are connected at their bases to theunderlying ePTFE structure. The islands are distinguishable from theunderlying nodes and fibrils because of their much larger size. Thelargest length dimension of the islands is at least twice that of thesame dimension of the underlying nodes. This length difference can evenexceed 100 times that of the underlying nodes. Further, the morphologyof the islands tends to distinguish them from the underlying ePTFEstructure. This island structure is unique to the surface of the articleand is not present below the surface.

The morphology of the PTFE structures of the present invention may alsovary widely with respect to the number of islands present on a givensurface area. In many cases, the islands are large and notinterconnected. In other embodiments, the islands are interconnected andmay appear as a porous covering or web atop the ePTFE structure. Giventhe expanse of the web, its size greatly exceeds that of underlyingnodes.

The unique character of the present articles and processes enable theformation of improved products not seen to date. For example, PTFEfibers can be made according to invention having improved performance insuch areas as dental floss, fishing line, sutures, and the like. PTFEarticles in membrane, tube, sheet and other forms can also provideunique characteristics in finished products. These and other uniquefeatures of the present invention will be described in more detailherein.

DETAILED DESCRIPTION OF FIGURES

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is perspective view of a cross-section of a fiber in accordancewith the present invention showing islands of PTFE above the surface ofthe underlying ePTFE structure.

FIG. 2 is perspective view of a fixture set-up for measuring mechanicalproperties of materials of the present invention as described in moredetail herein.

FIG. 3 is a schematic of the different comparative and inventive samplesand treatments referred to in the Examples and Comparative Examples.

FIGS. 4-6 are photomicrographs of the prior art precursor material usedin Example 1.

FIGS. 7-10 are photomicrographs of the inventive material made inaccordance with Example 1.

FIG. 11 is a photomicrograph of a prior art plasma-treated only materialmade in accordance with Comparative Example 1A.

FIG. 12 is a photomicrograph of a prior art heat-treated only materialmade in accordance with Comparative Example 1B.

FIG. 13 is a photomicrograph of the inventive material made inaccordance with Example 2.

FIGS. 14 and 15 are photomicrographs of the precursor material used inExample 3.

FIGS. 16-18 are photomicrographs of the inventive material made inaccordance with Example 3.

FIG. 19 is a graph showing the differential scanning calorimetry (DSC)scans comparing the features of the inventive materials with prior artmaterials, and described in more detail herein.

FIG. 20 is a photomicrograph of the precursor material used in Example4.

FIGS. 21 and 22 are photomicrographs of the inventive material made inaccordance with Example 4.

FIG. 23 is a photomicrograph of the precursor material used in Example5.

FIG. 24 is a photomicrograph of the inventive material made inaccordance with Example 5.

FIG. 25 is a photomicrograph of the precursor material used in Example6.

FIG. 26 is a photomicrograph of the inventive material made inaccordance with Example 6.

FIGS. 27 and 28 are photomicrographs of the precursor material used inExample 7.

FIGS. 29 and 30 are photomicrographs of the inventive material made inaccordance with Example 7.

FIG. 31 is a photomicrograph of the inventive material made inaccordance with Example 8.

FIGS. 32 and 33 are photomicrographs of the inventive material made inaccordance with Example 9.

FIG. 34 is a photomicrograph of the inventive material made inaccordance with Example 10.

DETAILED DESCRIPTION OF THE INVENTION

The PTFE articles of the present invention comprise islands of PTFEattached to an underlying ePTFE structure. No prior art materialexhibits these unique structures of PTFE islands attached to underlyingePTFE material. The identity of the island material can be confirmed bya variety of techniques. For instance, the island material can beassessed by scraping bits of just the island material off the surfacewith a razor blade, or by other suitable means, then performing athermal analysis on the sample. Differential Scanning Calorimetry (DSC)analysis of the islands, described later herein, indicates the absenceof a node and fibril structure.

Articles of the present invention are also unique in that the islands ofPTFE are of lower molecular weight than the PTFE of the underlying ePTFEstructure. This difference in molecular weight can be inferred frommeasuring and comparing the exotherms of the cooling curves obtainedfrom differential scanning calorimetry. Furthermore, the heating curvesindicate that the underlying ePTFE material possesses melt temperaturesat or about 327° C. and 380° C. The raised islands do not exhibit themelt temperature at or about 380° C.

The fundamental process for practicing the present invention is to firstsubject precursor ePTFE articles to a high-energy surface treatmentfollowed by a heating step to achieve the unique PTFE islands on thesurface of the underlying ePTFE material. Solely for convenience theterm “plasma treatment” will be used to refer to any high-energy surfacetreatment, such as but not limited to glow discharge plasma, corona, ionbeam, and the like. It should be recognized that treatment times,temperatures and other processing conditions may be varied to achieve arange of island sizes and appearances. For example, the PTFE surface canbe plasma etched in an argon gas or other suitable environment, followedby a heat treating step. Neither heat treating the ePTFE alone norplasma treating alone without subsequent heat treating results inarticles of the present invention.

This inventive process can be applied to a vast array of types andshapes of articles including, but not limited to, tubes, fibers,including but not limited to twisted, round, flat and towed fibers,membranes, tapes, sheets, rods, and the like, each possessing any of avariety of cross-sectional shapes. Depending on the morphology of theprecursor ePTFE material, the appearance of the islands can varysignificantly, and the process produces a more dramatic effect incertain precursor materials. For example, larger islands appear to beproduced in precursor materials possessing long fibrils and small nodeswhen processed in accordance with the teachings of the presentinvention.

In a further embodiment, the present invention also includes the step offilling just the surface of ePTFE with other materials. Filler particlescan be applied to the surface of the ePTFE article after the plasmatreatment step, before the heat treatment step. This process is referredto as surface filling, as distinguished from conventional means offilling the pores of porous ePTFE articles, which may include suchtechniques as blending or co-coagulation of the filler material withPTFE, impregnating the pores with filler, and altering the surface thenbonding other materials to that surface. The particles were primarilycontained within the islands as opposed to lying on the surface, as theywere prior to the heat treating step.

Articles of the present invention possess surprising and valuablefeatures heretofore unobtainable. In one embodiment, dental flossmaterials consisting essentially of PTFE are found to have significantlyincreased grippabililty and abrasive characteristics. Grippabilityrefers to the ability to firmly grip the floss during use such that itdoes not slide between the user's fingers. The abrasiveness provides theuser with an improved cleaning sensation, if not with improved cleaning,as well. These characteristics have not been realized to this degree inconventional PTFE floss materials.

The abrasiveness feature affords the creation of articles consistingessentially of PTFE that possess all of the advantages of PTFE andePTFE, without being lubricious. Lubricity is not a desirable feature inall applications.

Surprisingly, articles of the invention can simultaneously exhibitincreased abrasiveness evidenced by an increased drag coefficient andimproved abrasion resistance, as evidenced by improved durability inabrasion tests. Durability tests described herein quantify the frayresistance of articles. Even though the precursor material is subjectedto a plasma treatment step that would otherwise be expected by one ofskill in the art to compromise the abrasion resistance of the article,by virtue of the subsequent heat treating step, the inventive article issurprisingly more abrasion resistant than the precursor article. Thisdegree of abrasion resistance had heretofore only been achieved withePTFE floss materials with bulk densities less than about 0.8 g/cc.

The abrasion resistance also is particularly useful in solving frayingproblems associated with ePTFE fibers, especially with ePTFE fishinglines.

The islands of PTFE have also been demonstrated to improve the knotholding strength of suture materials made in accordance with theinventive process.

The presence of the islands may also enhance bonding inventive articlesto other articles, especially perfluoropolymer articles, PTFE articlesin particular.

The present invention will be further described with respect to thenon-limiting Examples provided below.

Test Methods

Drag Resistance Test

Dynamic drag resistance was determined using a fixture 180 as shown inFIG. 2 using three 12.7 mm (0.50 inch) diameter cylindrical shaftsmounted on a rigid beam which was cantilevered from a standard tensiletester, Model 5567 from INSTRON Company (Canton, Mass.). The fixture armsupport 176 was drilled and reamed nominal 12.7 mm diameter (nominal0.500 inch diameter) for a running fit of three cylinders 170, 172 and174 (available from McMaster-Carr Supply Company, Dayton, N.J., PartNumber 8524-K24, off-white, G-7 Garolite Glass Silicon Rod materialnominal 12.7 mm diameter, and parted off at nominal lengths of 25 mm) inthe fixture arm support, which were secured using set-screws compressingradially on the cylinders at the cylinder-support interface. Thecylinders were secured such that they did not rotate during a testiteration and extended out of the test fixture approximately 17 mm. Allthree cylinders were parallel which each other and perpendicular withthe cantilever fixture arm support 176.

The surface roughness (R_(a).) of the three cylinders was measured bothaxially and radially using a Perthometer Model M4P (Feinpruef Perthen,GmbH, Postfach 1853, D-3400 Goettingen, Germany). R_(a) was measured inthe cylinder axial direction at 4 quadrants 90 degrees apart measuredusing a stroke 0.03 inch. For the R_(a) in the cylinder radialdirection, 3 to 4 measurements were taken using a 0.01 inch strokerandomly along the length of the cylinder. The results are presented inthe table below. R_(a) Measurements - Axial R_(a) Measurements -Cylinder Number (microinches) Radial (microinches) 1 93/122/102/10355/56/59 2 32/27/67/55 101/53/48/69 3 52/57/118/66 60/98/68/40 AverageR_(a): 74.5 64.3 Standard Deviation: 32.3 19.2

Before each sample was tested, the cylinders were removed from thefixture, completely submerged in a beaker containing 99.9% isopropanolalcohol for 1 minute, replaced in the test fixture and permitted to airdry completely.

The INSTRON 5567 tensile tester was outfitted with a one yarn styleclamping jaw suitable for securing filament samples during themeasurement in the mode of tensile loading. The jaw was connected to a100 Newton rated load cell (not shown) which was secured on the tester'scross-head. The cross-head speed of the tensile tester was 30.48 cm perminute, and the gauge length was 50 mm (measured from the tangent pointof the yarn clamp down to the tangent point of the test specimen restingagainst the first of the three cylinders 170). The fixture 176 wassecured to the tensile tester such that the test specimen secured in theclamping jaw was perpendicular to the axis of cylinder 170.

The test article was threaded around the three cylinders 170, 172 and174 in the manner depicted in FIG. 2. Consequently, the sample waswrapped halfway around cylinder 170 and a quarter of the way aroundcylinders 172 and 174. Hence, a total cumulative wrap angle of one fullwrap (i.e., 2π radians) was achieved.

The vertical distance between the center points of cylinders 170 and 172tangent points was 25.4 mm. The horizontal distance between the centerpoints of the same two cylinders was 12.7 mm. The horizontal distancebetween the center points of cylinders 172 and 174 was 360.4 mm.

Since the inventive material may be produced to provide islands on onlyone side of the material, the samples were all twisted so that the sameside contacted the surface of all three cylinders. This results inplacing a one turn twist in all test specimens between cylinders 170 and172. The test specimens had no twist between cylinders 172 and 174. A300 gram weight 186 was fixed to the end of the test specimen. Thelength of the test specimen extending past cylinder 174 and down to thesuspended 300 gram weight 186 was at least 110 mm, but no more than 510mm.

In order to determine drag resistance of samples, five samples longenough to conduct the test were randomly selected and tested. To beginthe test, the tensile tester cross-head was set to move upwards, thuscausing the 300 gram weight to move upwards as well. The test specimenslid over the three cylinders for at least a travel length of 80 mm, butno more than 510 mm. The load cell was connected to a data acquisitionsystem such that the load induced as the test specimen slid over thecylinders during the upward motion of the cross-head was recorded at arate of at least 10 data points per second. The data acquisition systemrecorded the corresponding cross-head displacement during the testing aswell. The drag resistance at each cross-head displacement was thencalculated by the following formula:e ^((δθ)) =T ₂ /T ₁, which reduces to: δ=[ ln(T ₂ /T ₁)]/θ,

-   -   where:    -   δ=Drag Resistance    -   θ=Cumulative Wrap Angle in Radians=2π radians    -   T₁=average input tension=300 grams    -   T₂=average output tension as recorded by data acquisition in        gram force    -   (Note: ln is the natural logarithm base on e=2.71828)

Data were obtained for displacements between 0 mm to 76mm. The dynamicdrag resistance was determined by using the arithmetic mean-calculateddrag resistance over the displacement between 25.4 and 50.8 mm.

Note that samples possessing a wax or other coating can be tested afterremoving the coating material. Wax coating, for example, can be removedby soaking the floss in a heated bath at 60 deg C. of reagent gradeisopropanol alcohol for 10 minutes and then wiping the wax away using asoft cotton cloth.

Knot Holding Capacity Test for Sutures

Samples were prepared in the following manner: A length of the samplesuture material was wrapped twice around a 2-inch diameter smoothsurfaced (for example, Delrin) cylinder. The ends were tied togetherusing 4 sliding throws, and one alternate-sliding throw to lock. Throwswere tensioned so that the knot was positioned against the cylinder. The“ears” (ears are the two free ends of the suture after the knot is tied)were trimmed to lengths between ⅛ and 3/16-inch. The sample was slippedoff of the cylinder and the loop was cut in half at a location oppositethe knot.

Samples were tested using an INSTRON Model 5500R testing machine at a200 mm/min cross-head speed and 229 mm gauge length. Yarn grips and a10-kg load cell were used. At least ten samples were tested and the peakforce results were averaged (regardless of whether peak force occurredby breaking or slippage of the knot). All samples were tested in thetemperature range of 22-24° C.

Island Height Measurement

Island height was measured from scanning electron micrographs oflongitudinal cross-sections of the samples. Individual values of islandheight were measured as the shortest distance from the node-fibril ePTFEstructure to the highest point of the overlying island. A line was drawnacross the top surface of the node-fibril structure adjacent to theisland. A perpendicular line was then dropped from the highest point onthe island to the line on the surface of the node-fibril structure.

The length of the dropped line is the island height. Measurements werepreferably taken from micrographs taken at sufficiently highmagnification to enable a clear determination of the height, taking intoaccount the magnification of the scale bar at the bottom corner of thefigure. Individual measurements were taken for five randomly chosenislands that were representative of all the islands. The reported islandheight value is the average of those five individual measurements.

Test Method for Determination of Crystalline Phases inPolytetrafluoroethylene Material Based on Differential ScanningCalorimetry

Differential Scanning Calorimetry (DSC) can be used to identify thecrystalline phases of polytetrafluoroethylene (PTFE). The presence ofendothermic peaks during a heating scan, at approximately 320-340° C.shows the typical melting phases of PTFE. In addition, an endotherm atapproximately 380° C. is a consequence of PTFE having been expanded,thereby creating a node-fibril structure. This peak (or endotherm) iswidely recognized to be indicative of the presence of fibrils in thetest sample.

This test was performed using a TA Instruments Q1000 DSC and TAInstruments standard aluminum pans and lids for Differential ScanningCalorimetry (DSC). A TA Instruments Sample Encapsulation Press (Part No.900680-902) was used to crimp the lid to the pan. Weight measurementswere performed on a Sartorius MC 210P microbalance.

Calibration of the Q1000 was by performed by utilizing the CalibrationWizard available through the Thermal Advantage software supplied withthe device. All calibration and resulting scans were performed under aconstant helium flow of 25 ml/min.

Samples were prepared by either cutting pieces (6 mm or smaller) offiber or by loading already prepared surface and core material using ascraping method (described elsewhere herein). One pan and lid wereweighed on the balance to 0.01 mg precision. The sample material wasloaded into the pan and also recorded to 0.01 mg precision, with samplesranging from slightly under 1.0 mg for surface scraping samples tonearly 3.0 mg for some fiber samples. These values were entered into theThermal Advantage control software for the Q1000. The lid was placed onthe pan and was crimped using the press. Care was taken to ensure thatno sample material was caught in the crimp between the lid and the pan.A similar pan for reference was prepared, with the exception of thesample article, and its weight was also entered into the software. Thepan containing the sample article was loaded onto the sample sensor inthe Q1000 and the empty pan was loaded onto the reference sensor. Thesamples were then subjected to the following procedure:

-   -   1: Equilibrate at −30.00° C.    -   2: Ramp 10.00° C./min to 400.00° C.    -   3: Mark end of cycle 0    -   4: Isothermal for 5.00 min    -   5: Mark end of cycle 0    -   6: Ramp 10.00° C./min to 200.00° C.    -   7: End of method

Data were analyzed, unaltered, using Universal Analysis 2000 v.4.0 Cfrom TA Instruments. Where data were being analyzed qualitatively (forthe presence and temperature location of peaks), scans run under T4Pmode were used. In the case of quantitative interpretation ofcrystallization peaks (specifically, for the measurement of enthalpy),scans were run under T1 mode.

Tensile Break Load and Matrix Tensile Strength (MTS) for MembraneExamples

Tensile break load was measured using an INSTRON 5567 tensile testmachine equipped with flat-faced grips and a 10 kN load cell. The gaugelength was 2.54 cm and the cross-head speed was 25.4 cm/min. The sampledimensions were 6.35 cm×0.635 cm. For longitudinal MTS measurements, thelarger dimension of the sample was oriented in the machine (also knownas the down web) direction. For the transverse MTS measurements, thelarger dimension of the sample was oriented perpendicular to the machinedirection, also known as the cross web direction. Each sample wasweighed using an A&D scale, (Milpitas, Calif.), Model #FR-300, then thethickness of the samples was taken using the Heidenhain thickness gaugeModel #MT-60M (Schaumburg, Ill.). The samples were then testedindividually on the tensile tester. Five different sections of eachsample were measured. The average of the five break load (i.e., the peakforce) measurements was used. The longitudinal and transverse MTS werecalculated using the following equation:MTS=(break load/cross-section area)*(density of PTFE)/bulk density ofthe porous article),wherein the density of PTFE is taken to be 2.2 g/cc.MTS Calculation and Tenacity Measurement for Fiber and Suture Examples

For fiber materials, matrix tensile strength was derived from tenacityvalues. Tenacity was calculated using break load and sample weight data.Prior to tensile testing, the fiber denier was determined by weighing a9 m length sample of the fiber using an analytical balance (model AA160,Denver Instruments. Inc., Göttingen, Germany). The mass of the fiberexpressed in grams was multiplied by 1000 to arrive at the denier value.The 9 m long fiber sample was cut into five lengths for subsequent breakload testing. Tensile testing was conducted at ambient temperature on anINSTRON 5567 tensile test machine equipped with fiber grips and a 10 kNload cell, set to a sample length of 269 mm. The sample was loaded intothe grips and clamped. The break load was recorded as the grips moveapart at a speed of 254 mm/min. The tenacity of each fiber sample(expressed in grams/denier) was calculated by dividing the break load(expressed in grams) by the denier value of the fiber. The tenacityvalues for five samples were calculated and then averaged. Matrixtensile strength was then calculated by multiplying the tenacity value(in grams/denier) by 26,019.

Density Measurement

Fiber density was determined using one of two techniques. For fiberdensities greater than 1, the “principle of buoyancy,” or Archimedesprinciple, was used, which states that a body immersed in a fluid willbe subjected to a buoyancy force equal to the weight of the displacedfluid. Buoyancy force, or the weight of the displaced fluid, iscalculated from the initial fiber sample mass and the fiber sample massduring full immersion in the fluid. From the mass of the displaced fluidand the fluid density, the fluid volume displaced can be calculated andrepresents the total volume of the fiber. Using the initial “dry” massof the fiber and the fiber volume, the fiber sample density can becalculated.

A Duran glass volume standard was used to determine water density. Thisglass standard was certified to have a volume of 10±0.001 cubiccentimeters (cc). During the experiment, the room temperature wasrecorded at 71° F. (22° C.). The glass standard was placed on aMettler-Toledo AG204 series balance equipped with an integral immersiondensitometer, previously tared to zero, and the mass was noted at30.0409 g. A support was then placed over the balance base to allow adeionized water container to be placed over, but not in contact with,the balance. A support crucible was then suspended from the center ofthe balance into the water container and not allowed to contact thesides of the container. Any air bubbles attached to the crucible wereremoved by gentle agitation. The balance was then tared to zero. Theglass standard was then carefully placed on the crucible and fullyimmersed in the water container, avoiding contact with the sides of thecontainer. Any air bubbles attached to the glass standard afterimmersion in the water container were removed by gentle agitation of theglass standard on the crucible. The mass of the fully immersed glassstandard was noted at 20.0465 g. The density of water was calculated asfollows:buoyancy mass for the 10 cc glass standard=30.0409 g −20.0465 g=9.9944 gwater density=9.9944/10 cc=0.9994 g/cc.

All fibers with a density greater than 1 were tested using the followingprocedure. A fiber sample was placed on a Mettler-Toledo AG204 seriesbalance equipped with an integral immersion densitometer, and the masswas noted in grams (A).

As described above in the density determination of water, a support wasplaced over the balance base to allow a water container to be placedover but not in contact with the balance. A support crucible was thensuspended from the center of the balance into the water container andnot allowed to contact the sides of the container. Any air bubblesattached to the crucible after immersion in the water container wereremoved by gentle agitation. The balance was then tared to zero. Thefiber sample was then carefully placed on the crucible and fullyimmersed in the water container avoiding contact with the sides of thecontainer. Any air bubbles attached to the fiber after immersion in thewater container were removed by gentle agitation of the fiber on thecrucible. The mass of the fully immersed fiber was noted in grams (B).The density of the fiber sample was calculated as follows:fiber sample density (g/cc)=A/((A−B)/0.9994).

For fiber densities less than 1, the fiber volume was calculated fromthe average thickness and width values of a fixed length of fiber andthe density calculated from the fiber volume and mass of the fiber. Forfibers with a density less than 1, a 1.8 meter length of fiber wasplaced on an A&D FR-300 balance and the mass noted in grams (C). Thethickness of the fiber sample was then measured at 4 points along thefiber using a Heindenhain thickness gauge. The width of the fiber wasalso measured at 4 points along the fiber using a graduated eyepiecefrom Edmund Scientific Co. Average values of thickness and width werethen calculated, and the volume of the fiber sample was determined (D).The density of the fiber sample was calculated as follows:fiber sample density (g/cc)=C/D.Dimensional Measurements

Thickness was measured between the two plates of a Mitutoyo/MACmicrometer, unless indicated otherwise. Three different sections weremeasured on each sample. The average of the three measurements was used.

Diameter was measured using a single beam laser measuring device(LaserMike optical micrometer Model Number 60-05-06). Five differentsections were measured on each sample. The average of the fivemeasurements was used.

Width was measured using a digital caliper. Three different sectionswere measured on each sample. The average of the three measurements wasused.

Scraping Procedure

Scrapings of the islands of PTFE for DSC analysis were obtained in thefollowing manner. A portion of the sample was wrapped around a glassslide and positioned such that the islands faced upwards, then the endswere taped to the slide to prevent the sample from moving. Only theislands were scraped from the sample using fresh razor blades, with theaid of magnification (20-30× under a stereoscope). To ensure that onlyisland material was collected, it was visually confirmed that islandmaterial remained in each section from which scrapings were taken. Thisvisual confirmation ensured that scrapings did not extend into theunderlying node and fibril structure. Multiple samples were scraped tocollect island material until approximately 1 mg of scrapings was sogathered for DSC analysis.

Fiber Fray Test Method Description

Fiber samples were tested using the fixture in FIG. 2 used for the DragResistance Test, described earlier, which provides the details of thisfixture. Before each sample was tested, the cylinders were removed fromthe fixture, completely submerged in a beaker containing 99.9%isopropanol alcohol for 1 minute, replaced in the test fixture andpermitted to air dry completely.

The test article was threaded around the three cylinders 170, 172 and174 in the manner depicted in FIG. 2. Consequently, the sample waswrapped halfway around cylinder 170 and a quarter of the way aroundcylinders 172 and 174. Hence, a total cumulative wrap angle of one fullwrap (i.e., 2π radians) was achieved. The sample did not have any twistsbetween cylinders.

An INSTRON Model 5567 tensile tester outfitted with one yarn styleclamping jaw was used. The gauge length was 50 mm (measured from thetangent point of the yarn clamp down to the tangent point of the testspecimen resting against the first of the three cylinders 170). Thefixture 180 was secured to the tensile tester such that the testspecimen secured in the yarn style clamp was perpendicular to the axisof cylinder 170.

A 400 gram weight 186 was fixed to the end of the test specimen by tyinga looped knot around a 400 gram weight. The length of the test specimenextending past cylinder 174 and down to the suspended 400 gram weight186 was at least 150 mm. The tensile tester pulled the sample over thethree cylinders a distance of 50.8 mm at a cross-head speed of 50.8cm/min and then returned to its starting position to complete one cycle.Five consecutive cycles were run per sample.

The tested portion of the sample was marked by securing a piece of tapeon the sample 12 mm past cylinder 170 toward the yarn style jaw andsecuring another piece of tape on the sample 63 mm past cylinder 172toward cylinder 174.

The test method should be modified for fibers that do not have enoughtensile strength to survive the test. If any of the desired number ofsamples break during the five cycles the weight should be lowered by 100gram increments and the test should be started over until a weight isarrived at that does not cause any of the desired number of samples tobreak during the five cycles.

Upon completion of the test, the test samples were examined between thetwo pieces of tape for evidence of hairing. A hair is any part of thesample that has become frayed from the sample but is still attached atone end. Examination of the surface of the sample was performed usingeither a light ring with a 2× magnification lens or with a microscope(10× magnification). A caliper was used to measure the length of thehair, i.e., the length from the free end of the hair to the point wherethe hair is attached to the rest of the sample. The choice ofmagnification used, if any, is dependent on the ability to accuratelydetect and measure the length of the hairs.

The Fiber Fray Score for each sample was calculated from the length ofthe hairs coming off of the samples by the following equation:Fiber Fray Score=sum of the lengths in millimeters of the hairsFishing Line Fray Test

The fishing line to be tested was cut to a length of about 7.62 meters.One end of the sample to be tested was tied using a fisherman's doubleUni-knot to the free end of typical 12 lb test nylon fishing line thathad been spooled onto a Shakespeare Tidewater 10LA bait casting reel(Shakespeare Fishing Tackle, Inc., Columbia, S.C.). The length of thenylon line was such that it filled ¼ of the spool on the reel. The reelwas securely attached to the reel holder of a commercially availablefishing pole (7 ft Gold Cup Inshore rod rated for 12-25 lb lines and ¾-3oz. lures; Bass Pro Model GC171225, Springfield, Mo.). The pole wassecured at approximately a 10-degree angle. The pole was secured 20 mmbehind the last eyelet (toward the reel end of the pole) and 90 mm infront of the reel (toward the tip end of the pole). The tip wastherefore allowed to move and vibrate by the tensions of the line andthe inherent stiffness of the pole, as in a real fishing situation. Thepole was secured in such a way that the line did not touch the securingdevices during the test.

The other end of the sample fishing line to be tested was threadedthrough the pole guides and tied to a 16.83 cm diameter, about 50 mmwide, silicone coated take-up wheel in such a way that it did not slipor break during the test. The center of the wheel was located 15.24 cmbeyond the pole tip (in the horizontal direction) and 34.3 cm below thepole tip (in the vertical direction). The 50 mm wide part of the wheelwas positioned perpendicular to the fishing rod in such a way that theline could wind onto the 50 mm wide surface. This take-up wheel wasattached to a DC motor that accelerated to 1750 rpm in approximately ¼second. The rpm of the motor was measured with a digital hand tachometer(Ametek model 1726, Largo, Fla.) applied to the outside surface of thesilicone take-up wheel.

The reel was set to the casting, or open, position. The motor was turnedon and the line was wound onto the 50 mm wide part of the take-up wheel.This was intended to simulate casting the line during fishing. The motorwas turned off after the entire sample had been wound onto the take-upwheel. Pressure was applied to the exposed metal side of the spool byhand with a piece of PFTE tape and a sponge to prevent the spool fromover spinning while the take-up wheel was decelerating. The reel wasswitched to the closed or reeling position. An air drill (Matco ModelMT1889, Stow, Ohio) attached to the handle of the reel in order tore-spool the line was turned on. The drill re-spooled the line at a rateof 85 to 88 feet per minute as measured by a digital hand tachometer(Ametek model 1726, Largo, Fla.) on the silicon surface of the wheel andwith a back tension of 1800-2000 g applied to the wheel. The backtension was intended to simulate the resistance of a fish on the lineand was measured by placing a Saxl Tension Meter Model TR-4000(Tensitron, Inc., Harvard Mass.) onto the sample between the reel andthe first eyelet as the sample was being reeled up by the air drill. Acycle was complete once the sample fishing line was respooled on thereel, minus the amount strung through the rod and tied onto the wheel.The air drill was turned off. Each line was subjected to 5 such testcycles.

Upon completion of the test, the test samples were examined over theirentire length for evidence of hairs. A hair is any part of the line thathas frayed and become separated from the line, but is still attached atone end. Examination of the surface of the sample was performed usingeither a light ring with a 2× magnification lens or with a microscope(10× magnification). A caliper was used to measure the length of thehair, i.e., the length from the free end of the hair to the point wherethe hair is attached to the rest of the sample. The choice ofmagnification used, if any, is dependent on the ability to accuratelymeasure the length of the hairs.

A Fishing Line Fray Score for each sample was then calculated from thelength of the hairs coming off of the samples using the followingequation:Fishing Line Fray Score=sum of the lengths in millimeters of hairs over4 mm in length.Moisture Vapor Transmission Rate (MVTR)

The samples (measuring larger than 6.5 cm in diameter) were conditionedin a 23° C., 50%±2% RH test room. Test cups were prepared by placing 70grams of a Potassium Acetate salt slurry into a 4.5 ounce polypropylenecup having an inside diameter of 6.5 cm at the mouth. The slurry wascomprised of 53 grams of potassium acetate crystals and 17 g of water.The slurry was thoroughly mixed with no undissolved solids present andstored for 16 hours in a sealed container at 23° C. An expanded PTFEmembrane (ePTFE), available from W. L. Gore and Associates,Incorporated, Elkton, Md., was heat sealed to the lip of the cup tocreate a taut, leakproof microporous barrier holding the salt solutionin the cup. A similar ePTFE membrane was mounted taut within a 12.7 cmembroidery hoop and floated upon the surface of a water bath in the testroom. Both the water bath and the test room were temperature controlledat 23° C.

Samples to be measured were laid upon the floating membrane, and a saltcup inverted and placed upon each sample. The salt cups were allowed topre-condition for 10 minutes. Each salt cup was then weighed, invertedand placed back upon the sample. After 15 minutes, each salt cup wasremoved, weighed, and the moisture vapor transmission rate wascalculated from the weight pickup of the cup as follows: $\begin{matrix}{M\quad V\quad T\quad R\quad g\text{/}\left( {m^{2} \times} \right.} \\\left. {24\quad{hours}} \right)\end{matrix} = \frac{{Weight}\quad(g)\quad{water}\quad{pickup}\quad{in}\quad{cup}}{\begin{matrix}\left\lbrack {{Area}\quad\left( m^{2} \right)\quad{of}\quad{cup}\quad{mouth} \times} \right. \\{\left. {{Time}\quad({days})\quad{of}\quad{test}} \right\rbrack.}\end{matrix}}$The average of five tests was used.

EXAMPLES

In order to demonstrate the unique surfaces of the materials of thepresent invention as compared to prior art surfaces and treatments,surface and longitudinal cross-section scanning electron micrographswere taken, in many cases, for each of the following three “comparative”materials and for the inventive material of the present invention: (A)precursor material; (B) plasma-treated only material, (C) heat-treatedmaterial only, and (D) inventive material that was subjected to theunique combination of plasma treating then heat treating to effect aunique surface on the inventive material. FIG. 3 is a schematic, forreference only, of the different comparative and inventive samples andtreatments described in the following examples. Higher magnificationimages were taken in the same region that the low-magnification imageswere taken. Samples were thoroughly scanned to ensure that the imageswere representative of the sample.

Example 1

Precursor Material:

Expanded PTFE dental floss material made in accordance with theteachings of U.S. Pat. No. 5,518,012 was the precursor for the twocontinuous processing techniques performed in this example, describedbelow as (a) and (b). This dental floss was an ePTFE flat fiberpossessing the following properties: bulk density of 1.52 g/cc,thickness of 0.05 mm, width of 1.2 mm, and matrix tensile strength of81,401 psi in the length direction, drag resistance of 0.148 and FiberFray Score of greater than 200 (exact numbers were not calculatedbecause of the abundance of hairs). Representative scanning electronphotomicrographs of the precursor material, all taken at 500×magnification, appear in FIGS. 4 through 6. The dashed bars present atthe lower right of these and all other micrographs presented hereinindicate the magnification scale. For example, the distance between thefirst and last dash marks in FIG. 4 corresponds to a length of 100microns. The precursor material was produced by stretching PTFE overheated plates. FIGS. 4 and 5 show both of the surfaces of the precursormaterial, namely, the surface that did contact the plate and the surfacethat did not contact the plate, respectively. Islands of PTFE are notevident in either of these photomicrographs. FIG. 6, which shows across-section of the precursor material, also confirms the absence ofislands in the precursor material. These three photomicrographs of theprecursor material depict an ePTFE structure that is representative ofhighly longitudinally-expanded materials.

Experimental Procedures:

(a) Long lengths of the precursor material were first plasma treatedusing argon gas in conjunction with a Plasma Treatment System PT-2000P(Tri-star Technologies, El Segundo, Calif.). A T-section was affixed tothe end of the nozzle of the unit. Plasma treatment occurred within thestraight length of the T-section. The precursor floss material was fedthrough the straight section, which measured 59 cm long and 3.7 mm innerdiameter. The floss material was drawn through the unit at a linearspeed of 30 fpm, and the power was set between 2.1 and 2.2, per the“Plasma Current” display on the front of the unit. The argon flow ratewas set at about 25 SCFH. The plasma-treated material was next subjectedto a heat-treating step by passing it over a heated plate set to 390° C.at a line speed of 60 fpm. The length of the heated plate was 86 inches(2.2 m).

Photomicrographs of the plasma-treated, then heat-treated materialsappear in FIGS. 7 through 10. FIG. 7 was taken at 200× magnification,and FIGS. 8 through 10 were taken at 500×. FIGS. 7 and 8 are surfaceshots taken of the plate side of the material, FIG. 9 is a surface shottaken of the non-plate side of the material, and FIG. 10 is across-sectional photomicrograph. The surface images indicate the smooth,island-like appearance of the PTFE material on top of the node-fibrilstructure of the underlying ePTFE floss material. These imagesdemonstrate that the individual islands have a much larger surface areathan any of the nodes of the underlying node-fibril ePTFE structure. Theisland height was determined to be about 17 microns.

The inventive article had the following properties: bulk density of 1.52g/cc, longitudinal matrix tensile strength of 62,113 psi, width of 1.1mm, and thickness of 0.05 mm. The inventive material had a dragresistance of 0.196, which was consistent with the perception ofincreased grippability and improved cleaning sensation experienced uponhandling and using the inventive material. Three inventive samples weresubjected to the Fiber Fray Test and were found to have no visiblehairs, resulting in a Fiber Fray Score of 0.

(b) Another sample of the precursor material was processed in the sameway as described above in procedure (a), except that faster line speedsof 200 feet per minute for both the plasma treating and subsequent heattreating were employed. The resulting inventive material had a dragcoefficient of 0.192, and island height of 6 microns.

Comparative Example 1A

The same precursor material as described in Example 1, above, was usedin this comparative example. A long length of the precursor material wasplasma treated using argon gas in conjunction with a Plasma TreatmentSystem PT-2000P (Tri-star Technologies, El Segundo, Calif.). A T-sectionwas affixed to the end of the nozzle of the unit. Plasma treatmentoccurred within the straight length of the T-section. The precursorfloss material was fed through the straight section, which measured 59cm long and 3.7 mm inner diameter. The floss material was drawn throughthe unit at a linear speed of 30 fpm, and the power was set between 2.1and 2.2, per the “Plasma Current” display on the front of the unit. Theargon flow rate was set at about 25 SCFH.

This plasma treatment resulted in a material possessing the followingproperties: bulk density of 1.52 g/cc, thickness of 0.1 mm, width of 1.2mm, and matrix tensile strength of 69,998 psi. FIG. 11 is aphotomicrograph of this plasma-treated only material, showing a surfacedevoid of islands.

Comparative Example 1B

The same precursor material as described in Example 1, above, was usedin this comparative example. A long length of the precursor material wassubjected to a heat-treating step by passing it over a heated plate setto 390° C. at a line speed of 60 fpm. The length of the heated plate was86 inches (2.2 m). FIG. 12 is a photomicrograph taken at 500× of thenon-plate side of this heat-treated material. This image shows that thematerial surface is devoid of islands.

Example 2

The same precursor material as described in Example I was used in thisexample. The precursor material samples were subjected to the sameplasma treatment described in Example 1, part (a), then theplasma-treated samples were axially restrained and placed in a forcedair oven set to 335° C. for about 10 minutes.

Surface and longitudinal cross-section scanning electronphotomicrographs were obtained for this inventive material. FIG. 13 is asurface photomicrograph of the floss material sample taken at 1000×magnification. The islands that are characteristic of articles of thepresent invention are evident in this photomicrograph. As with theislands observed in Example 1, the island surfaces appear smooth and theindividual islands are of greater surface area than any of theunderlying nodes.

The inventive article had the following properties: bulk density of 1.46g/cc, longitudinal matrix tensile strength of 64,345 psi, width of 1.1mm, and thickness of 0.17 mm. The inventive floss material, when triedby several individuals, gave the perception of improved grippability andcleaning sensation compared to the precursor material.

Example 3

Precursor Material:

Expanded PTFE dental floss made in accordance with the teachings of U.S.Pat. No. 6,539,951 was the precursor material for this example. Thisdental floss consisted essentially of ePTFE and possessed the followingproperties: bulk density of 0.80 g/cc, thickness of 0.08 mm, width of1.9 mm, matrix tensile strength of 63,949 psi, and drag coefficient of0.172. Photomicrographs of the surface and cross-section, respectively,of this precursor material appear in FIGS. 14 (500×) and 15 (1000×).

Experimental Procedure:

For the present example, the precursor material was plasma-treated, thenheat treated in accordance with the steps described in Example 1, part(a). FIG. 16 (surface, 200×), FIG. 17 (surface, 500×), and FIG. 18(cross-section, 1000×) are photomicrographs of the microstructure of theinventive material. As with the prior examples, the individual islandsare seen to have a much larger surface area than any of the nodes of theunderlying node-fibril ePTFE structure, and the islands exhibit a smoothsurface. The inventive material had the following properties: bulkdensity of 0.82 g/cc, longitudinal matrix tensile strength of 36,707psi, width of 1.8 mm, and thickness of 0.08 mm.

The average island height for the inventive material was determined tobe about 13 microns. The drag coefficient for the inventive material wasmeasured to be 0.220, thus indicating that the inventive article wasmore grippable than the precursor article and had an improved cleaningsensation.

Differential Scanning Calorimetry (DSC) was used to determine whethermultiple crystalline phases of PTFE existed in the islands and in theunderlying core, or non-island, component of the material made in thisexample. Scrapings of the islands were taken by following the ScrapingProcedure described herein. FIG. 19 herein includes the DSC scans forthe inventive material as a whole, as well as for the scrapings aloneand the underlying core alone. The results are described in more detaillater herein, along with comparisons with Comparative Example 3A and 3Bmaterial scans.

Comparative Example 3A

The precursor material described in Example 3 was used for thiscomparative example. This precursor material was subject to the sameplasma treatment described in Comparative Example 1A.

Comparative Example 3B

The precursor material described in Example 3 was used for thiscomparative example. This precursor material was subject to the sameheat treatment described in Comparative Example 1B.

FIG. 19 shows six DSC heating scans for the inventive materials ofExample 3 (labeled (1), (2) and (3) on the figure), the precursormaterial for Example 3 (labeled (4)), and for Comparative Example 3A(labeled (5)) and 3B (labeled (6)). All samples were tested in themanner described in the Test Method for Determination of CrystallinePhases in Polytetrafluoroethylene Material based on DifferentialScanning Calorimetry. The curves were overlaid on the same graph andshifted on the y-axis for clarity. The curve corresponding to theinventive sample is labeled as (1). Islands from a section of thissample were scraped off the surface per the Scraping Procedure, and theheating scan for this island material is labeled (2). A scan was alsoprepared by obtaining core material from the center of the inventivematerial sample, ensuring that all island material was removed, and thecurve for this core material is labeled (3).

All but one of the scans in this FIG. 19 exhibit the approximately 380°C. peak in the heating curves. The only sample that did not exhibit thispeak was the island material obtained by scraping (scan (2)). Theabsence of this endotherm in this DSC curve indicates that the islandsdo not contain the node and fibril structure that is present in all ofthe other materials. This result is consistent with the absence ofdiscernable fibrils in the islands evidenced in the micrographs.

From the DSC cooling scan, the exothermic enthalpy (as expressed inunits of J/g) represented by the area of the peak at approximately 316°C. provides information regarding the molecular weight of the PTFE.Lower molecular weight PTFE has higher enthalpic values because thematerial can recrystallize more readily during cooling than highermolecular weight PTFE. The exothermic enthalpy of the core of theinventive material devoid of all islands, represented by the area of thepeak at approximately 316° degree C., was 33.5 J/g. The exothermicenthalpy of the island scrapings had an exothermic enthalpy, representedby the area of the peak at approximately 316° C., of 60.5 J/g. Thehigher exothermic enthalpy of the islands as compared to the coreindicated that the islands were comprised of lower molecular weight PTFEthan the core.

Example 4

Expanded PTFE fiber was obtained (Part Number V112765, available from W.L. Gore and Associates, Inc., Elkton, Md.), and two such fibers weretwisted together to provide the precursor material for this example. Theprecursor material possessed the following properties: bulk density of1.29 g/cc, longitudinal matrix tensile strength of 138,278 psi, anddiameter of 0.483 mm. FIG. 20 (100×) is a photomicrograph of the surfaceof the precursor material.

In this example, the precursor material was plasma treated and heattreated in the same manner as described in Example 1, part (a), exceptthat the plasma treatment line speed was set at 100 fpm, and the heattreatment was performed over a series of three heated plates, measuring9 feet total, all set to 440° C. to effect a modest amount of shrinkageby applying an overall stretch ratio of 0.92:1.

The inventive article had the following properties: bulk density of 2.17g/cc, longitudinal matrix tensile strength of 92,285 psi, diameter ofapproximately 0.41 mm. The cross-section of the article was of oblongshape. The island height was determined to be about 6 microns. FIG. 21(100×) and FIG. 22 (1000×) are surface photomicrographs of the inventivematerial. Both figures show raised, smooth-surfaced islands.

In addition, three samples of the inventive fishing line material weresubjected to the Fishing Line Fray Test, and all of the inventivefishing lines exhibited only small hairs ranging from 0.5 mm to 6 mm inlength. Fishing Line Fray Scores for these three samples were 4, 5, and10, respectively.

Comparative Example 4A

The precursor material described in Example 4 was used for thiscomparative example. Comparative fishing line material were made by heattreating the precursor over a series of three heated plates, all set to440° C. to effect a modest amount of shrinkage by applying an overallstretch ratio of 0.92:1.

Three comparative fishing line samples were subjected to the FishingLine Fray Test. Each of the three samples had many hairs of varyinglengths from 0.5 mm to as long as 38 mm, with at least 10 hairs over 10mm in length and at least two hairs over 20 mm in length. The FishingLine Fray Scores for these samples were all over 160 (exact numbers werenot obtained because of the abundance of hairs).

Example 5

The precursor material for this example was expanded PTFE suturematerial possessing the following properties: bulk density of 1.13 g/cc,longitudinal matrix tensile strength of 56,382 psi, and diameter of 0.3mm. FIG. 23 is a photomicrograph taken at 200× of the precursormaterial.

This precursor material was plasma treated in the same manner asdescribed in Example 1(a); however, the subsequent heat treating wasperformed in a continuous manner, drawing the plasma treated articlethrough a 92-inch-long forced air oven set to 415° C. at a line speed ofabout 15 ft/minute. The resulting inventive article had the followingproperties: bulk density of 1.07 g/cc, longitudinal matrix tensilestrength of 44,986 psi, and a diameter of 0.33 mm. The island height wasdetermined to be about 11 microns. FIG. 24 is a photomicrograph taken at200× of the inventive material.

FIGS. 23 and 24 demonstrate the difference in the surface appearancebetween the precursor and inventive materials, respectively. Theinventive material clearly exhibits the raised islands of PTFE, in whichthe islands are smooth and are of greater size than the nodes of theunderlying structure. As with all of the images included herein, sampleswere thoroughly scanned to ensure that the images were representative ofthe sample.

The inventive materials were subjected to the Knot Holding CapacityTest, and the knotted inventive article retained 59% of its materialpeak force, and the inventive suture broke at the knot in 70% of thecases.

For comparison purposes, a sample of the knotted precursor suturematerial, when subjected to the Knot Holding Capacity Test, retainedonly 27% of its material peak force and in each test the knot slippedwithout the suture breaking.

Example 6

The precursor material for this example was an expanded PTFE fibermaterial, suitable for use as a suture, having a diameter of 0.023 mm.FIG. 25 is a photomicrograph taken at 500× of the precursor material.

The precursor material was first plasma treated using argon gas inconjunction with a Plasma Treatment System PT-2000P (Tri-starTechnologies, El Segundo, Calif.). A T-section was affixed to the end ofthe nozzle of the unit. Plasma treatment occurred within the straightlength of the T-section. The precursor floss material was fed throughthe straight section, which measured 59 cm long and 3.7 mm innerdiameter. The floss material was drawn through the unit at a linearspeed of 5 fpm, and the power was set at 1.8, per the “Plasma Current”display on the front of the unit. The argon flow rate was set at about25 SCFH. The plasma-treated material was next restrained from shrinkingby tying it to a metal frame, then subjected to a heat-treating step byplacing it in a forced air oven set to 335° C. for 10 minutes. Islandsof PTFE are evident on the inventive material, as shown in FIG. 26,which is a photomicrograph taken at 500×.

Example 7

The precursor material for this example was an expanded PTFE membranepossessing the following properties: moisture vapor transmission rate of68,149 g/m²-day, thickness of 0.023 mm, bulk density of 0.80 g/cc,longitudinal matrix tensile strength of 8,740 psi, and transverse matrixtensile strength of 15,742 psi. FIGS. 27 and 28 are photomicrographs ofthe surface and the cross-section, respectively, of the precursormembrane, both taken at 2000× magnification.

The membrane material was then processed to provide articles of thepresent invention. The precursor membrane was subjected to a plasmatreatment using argon gas by passing the membrane through an atmosphericplasma treatment unit set to a power of 2.5 kilowatts. The membrane waspassed through the unit at a speed of 5 meters per minute, and the argongas flow rate was 50 liters per minute. The plasma-treated membrane wassubsequently restrained from skrinking by securing it on a pin frame andheat treated in a forced air oven set to 335° C. for about 10 minutes.

The resulting inventive material had the following properties: bulkdensity of 0.81 g/cc, longitudinal matrix tensile strength of 10,070psi, transverse matrix tensile strength of 14,375 psi, and thickness of0.023 mm. FIGS. 29 and 30 are surface and cross-sectionalphotomicrographs, respectively, of the inventive material taken at2000×, showing smooth, raised islands. The island height of theinventive material was determined to be about 3 microns.

Example 8

The same precursor membrane material described in Example 7 was used forthis example. The precursor was processed in the same manner describedin Example 7, except that round silica particles (Admatechs, ProductNumber SO-E2, Seto, Japan) were applied to the surface of theplasma-treated membrane by sprinkling, then the particles were spreadout by a gloved hand to form a thin, substantially even coating on themembrane prior to the heat-treating step.

A photomicrograph of the surface of the inventive article taken at 2000×appears in FIG. 31. Upon examination of the photomicrograph, it wasobserved that the raised islands contained silica particles.

Example 9

The precursor membrane material described in Example 7 was used for thisexample. The membrane was processed in the same manner as described inExample 7, except that a mask material comprising a polyester film tapewith a rubber adhesive (₃M™ Polyester Protective Tape 335, MinnesotaMining and Manufacturing, Inc., St. Paul, Minn.) having a pattern ofsubstantially regularly-spaced holes was taped to the surface of theprecursor material prior to the plasma-treatment step. The mask wasremoved after the plasma-treatment, but prior to the heat treatmentstep.

FIGS. 32 and 33 are surface shots taken at 70× and 2000×, respectively,of the resulting article of this example. FIG. 32 shows the dot patterneffected by masking the PTFE during the plasma-treating process.Specifically, the areas that appear as dots (darker) 501 are areas thatwere plasma-treated then heat-treated; hence, these regions wereprocessed in accordance with the present invention. The masked (lighter)regions 502 were subjected only to heat treatment. A representativehigher magnification image of the boundary between the masked 502 andunmasked 501 regions is presented in FIG. 33. Note the smooth islands503 on the plasma-treated and heat-treated area, as compared to themasked region 502.

Example 10

A precursor material comprising expanded PTFE fiber which had never beensubjected to amorphous locking temperatures was obtained having thefollowing properties: bulk density of 1.2 g/cc, longitudinal matrixtensile strength of 71,000 psi, width of 1.2 mm, and thickness of 0.2mm.

The precursor material was processed in the same manner as part (a) ofExample 1. The resulting inventive article had the following properties:bulk density of 1.4 g/cc, longitudinal matrix tensile strength of 64,400psi, width of 0.9 mm, and thickness of 0.2 mm. A photomicrograph takenat 500× of the surface of the resulting inventive material appears inFIG. 34. This figure shows the raised islands of PTFE on the material,thus demonstrating that articles of the present invention are createdeven with ePTFE precursor materials which have not been subjected toamorphous locking temperatures.

While the invention has been disclosed herein, in connection withcertain embodiments and detailed descriptions, it will be clear to oneskilled in the art that modifications or variations of such detail canbe made without deviating from the gist of the invention and suchmodifications or variations are considered to be within the scope of theclaims herein below.

1. A process for forming a PTFE article comprising: subjecting anexpanded PTFE article to a plasma treatment; and subjecting the plasmatreated material to a heat treatment.